GaAs-based heterojunction bipolar transistors (HBTs) employing heavily p-doped material (e.g. about 4×1019 cm−3) in the base layer have developed into a key device technology utilized in a variety of commercial and military applications, particularly as power amplifiers for wireless handsets (see, for example, U.S. Pat. No. 6,750,480, U.S. Patent Application Publication No. 2002/0163014, and International Application Publication Nos. WO 02/043155 and WO 03/088363, the entire teachings of which are incorporated herein by reference). The physical properties of the base layer fundamentally control many of the key DC and RF device characteristics. One of the most critical properties is the base transit time (τb,tr). The base transit time directly limits the peak DC current gain, or beta (β), of HBTs, which in the limit of neutral base recombination can be expressed as:βnbr≅νbτrec/wb=τrec/τb,tr  (1)where νb is the average minority carrier velocity across the base, τrec is the minority carrier lifetime in the base, and wb is the base thickness. Base transit time also directly impacts bipolar transistor RF properties. The cutoff frequency ft is well known to be inversely related to the sum of the transit and charging times:½πft=τb,tr+τcscl+(τe+τc)  (2)where τcscl is the collector transit time, and τe, τc are the emitter and collector charging times. The expected maximum frequency of operation (fmax) is related to both ft and the base resistance (Rb) which is, in turn, related to the base sheet resistance (Rsb):fmax=(ft/8πRbCbc)1/2  (3)where Cbc is the base-collector capacitance. The power gain of a bipolar transistor circuit is directly related to fmax and β. Equations (1)-(3) clearly illustrate the benefit of reducing base transit time. The collector current density (Jc) of a bipolar transistor can also be enhanced via a decrease in the base transit time:Jc=qnavgvavg=qnavgwb/τb,tr  (4)where navg is the average carrier concentration in the base layer.
Therefore, reducing the base transit time, especially if accomplished without degrading other device properties such as Rsb, is of great benefit to enhancing the performance of bipolar transistor devices and circuits. Consequently, further improvements in the fabrication of semiconductor materials of bipolar transistors would reduce the base transit time without degrading Rsb, and thereby improve the performance of the devices.